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How are images on the retina different from the actual object being viewed?
Inverted and reversed
The fovea represents what percent of visual space? And what percent of optic nerve fibers and primary visual cortex cells does it correspond to?
Central 1-2 degrees of visual space. 50 percent of optic nerve fibers and 50 percent of primary visual cortex neurons
Where is the optic disc related in relation to the fovea? Where is the blind spot?
Optic disc is 15 degrees nasal to the fovea. The blind spot is thus 15 degrees temporal to the fovea
What is the ratio of rods to cones in the retina?
20 to 1
Function of rods?
Vision in low level lighting (very sensitive)
Function of cones?
High spatial and temporal resolution; Color detection
What is the receptive field of a neuron?
Portion of the visual field where light causes excitation or inhibition of the cell
Photoreceptors synapse onto what; which synapses onto what; which forms what?
Bipolar cells; Retinal ganglion cells; Axons join to form the optic nerve. Note that these connections occur in a vertical manner thru the retina
What are the 2 retinal ganglion cell types?
1) M (or Pa or A) cells: large receptive fields; respond to gross features and movements; project to the Magnocellular layers of LGN; 2) P (or Pb or B) cells: small receptive fields; respond to fine detail and color; project to Parvocellular layers of LGN.
What is the function of horizontal cells or amacrine cells?
Form lateral inhibitory or excitatory connections with nearby bipolar and ganglion cells; this results in bipolar and retinal ganglion cells having a center-surround receptive field
What are the 2 types of center-surround receptive fields?
1) On-center: cells are excited by light in the center of the receptive field and inhibited by light in the surrounding area; 2) Off-center: cells are inhibited by light in the center of the field and excited by light in the surrounding area
Why is the optic nerve not actually a nerve?
It lies in the CNS and not the PNS. Retinal bipolar cells are analogous to the primary somatosensory neurons (DRG) whereas retinal ganglion cells are like secondary somatosensory neurons that project to the thalamus.
What fibers cross in the optic chiasm?
The nasal fibers for each eye (corresponding to the temporal field of each eye)
Lesions where will produce monocular visual field defects?
Anterior to optic chiasm
Lesions where will produce homonymous visual field defects?
Posterior to optic chiasm
Describe the path of the optic tracts to reach the LGN of the thalamus?
They wrap around the lateral surface of the midbrain
What are the 2 paths the retinal ganglion axons can takes?
1) Lateral geniculate nucleus and then to primary visual cortex: involved in visual discrimination and perception; 2) Extrageniculate - to pretectal area and superior colliculus and then to brainstem areas; then to pulvinar and lateral posterior nucleus of thalamus; then to association cortex or brainstem : involved in visual attention and orientation
How many layers does the LGN have and what are they?
6 layers: 1+2 are magnocellular layers (relay info from M retinal ganglion cells; each layer corresponds to a different retina); 3-6 are parvocellular layers (relay info from P retinal ganglion cells; layers alternate which retina is represented)
Describe the path of the optic radiations from the thalamus to primary visual cortex?
Sweep over and lateral to the temporal horn and atrium of the lateral ventricle
Describe the path of the inferior optic radiations to the primary visual cortex? What is this path called and what information is carried in this path?
The fibers arc forward into the temporal lobe and project to the inferior bank of the calcarine fissure. Meyer\\\'s loop. Carries info from the inferior retina (or the superior visual field)
Temporal lobe lesions can cause what kind of visual defect?
Contralateral homonymous superior quadrantoanopia (pie in the sky)
Describe the path of the superior optic radiations to the primary visual cortex? What information is carried in this path?
Fibers pass under the parietal lobe and project to the superior bank of the calcarine fissure. Carries info from the superior retina (or the inferior visual field)
Parietal lobe lesions can cause what kind of visual defect?
Contralateral homonymous inferior quadrantoanopia (pie on the floor)
Where is the primary visual cortex located?
Calcarine fissure of the occipital lobe
Describe the retinotopic organization of the primary visual cortex
Remember that the primary visual cortex corresponds to the contralateral half of the visual field. The upper bank corresponds to the inferior quarter of that visual field whereas the lower bank corresponds to the superior quarter of that visual field. The foveal (center) region is represented closest to the occipital pole. The more peripheral regions are mapped as you move anteriorly along the calcarine fissure.
Most input to the primary visual cortex arrives at what cortical layer?
Layer 4
Describe the path that information about movement and gross spatial features takes?
Starts in magnocellular retinal ganglion cells. Goes to M layers of the LGN. Projects to layer 4C-alpha of visual cortex
Describe the path that information about fine spatial features takes?
Starts in parvocellular retinal ganglion cells. Goes to P layers of the LGN. Projects to layer 4C-beta of visual cortex
Describe the path that information about color takes?
Starts in parvocellular retinal ganglion cells. Goes to P layers and interlaminar zones of the LGN. Projects to regions called blobs in layers 2-3 of visual cortex
Primary visual cortex is also known as Brodmann area number?
Area 17
Where does information go from the primary visual cortex?
To extrastriate regions of visual association cortex and parieto-occipital and occipito-temporal cortex
What are the two streams of higher-order visual processing after the primary and secondary visual cortex?
Dorsal pathways: project to parieto-occipital association cortex and answer the question WHERE? By analyzing motion and spatial relationships between objects and between the body and visual stimuli; Ventral pathways: project to occipito-temporal association cortex and answer the question WHAT? by analyzing colors and faces and letters and other visual stimuli
Describe the columnar organization of the visual cortex
Inputs from each eye terminate in different alternating bands of cortex (1mm wide) called ocular dominance columns. There are also orientation columns (which are vertical columns in which all cells respond to the same orientation (ie a line or edge at a specific angular orientation)). As you move horizontally across the cortex the oreintation selectively changes continuously so a complete 180 degree sequence occurs over 1mm.
Why do visual disturbances during migraines often involve zigzagging lines?
Possibly because of activation of alternating orientation columns in the primary visual cortex
Where is the lesion if a pt has formed visual hallucinations (ie sees people; animals; or complex scenes)?
Inferior temporo-occipital visual association cortex
What is a crude way of testing visual field in an uncooperative or lethargic patient?
Blink to threat
What is a homonymous visual field defect?
Same region of the field is affected in both eyes
Lesions to the upper bank of the calcarine fissure cause what visual field defect?
Contralateral inferior quadrantanopia
Lesions to the lower bank of the calcarine fissure cause what visual field defect?
Contralateral superior quadrantanopia
Lesions to the entire visual cortex cause what visual field defect?
Contralateral Homonymous hemianopia
Give a few reasons why macular sparing often occurs with lesions in the visual pathway?
1) Fovea has a very large representation for its size - remember it accounts for 50 percent of neurons in primary visual cortex! 2) MCA and PCA may provide collateral flow to representation of macula in occipital pole
What are the 3 main causes of impaired blood flow thru the ophthalmic artery?
1) Embolism 2) Stenosis due to DM; HTN; or elevated ICP; 3) Vasculitis eg temporal arteritis
Describe the sxs of amaurosis fugax and why sxs might occur in this fashion
Visual field loss like a shade coming down or up over the eye. Perhaps this occurs because the retinal artery has 2 major branches- one covering the upper half and one covering the lower half of the retina.
Infarcts of the LGN cause what visual field defect? What other sxs can be seen?
Contralateral homonymous hemianopia; Can also see contralateral hemiparesis or hemisensory loss due to involvement of nearby posterior limb of internal capsule and thalamic somotosensory radiations
What does a bilateral altitudinal scotoma suggest?
Vertebrobasilar insufficiency causing bilateral infarcts or TIAs (recall that the basilar artery supplies both PCAs)
What are the clinical features of optic neuritis?
Eye pain (esp with movement); Monocular central scotoma (or even complete monocular visual loss); Decreased visual acuity; Afferent pupillary defect; Red desaturation; Increased latency of visual evoked potentials
How do you test for red desaturation?
Ask pt to compare appearance of a bright red object seen with one eye vs the other eye. In pts with present or past optic neuritis the object appears duller in the affected eye
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